16. What is Graceful Shutdown and GoodBye message in EIGRP?
When an EIGRP process is shut down, router sends out “goodbye” messages to its neighbors. The neighbors can then immediately begin recalculating paths to all the destinations that went through that shutdown router without having to wait for the hold timer to expire.

17. How Passive Interface command works in EIGRP?
With EIGRP running on a network, the passive-interface command stops sending outgoing hello packets, hence the router cannot form any neighbor relationship via the passive interface. This behavior stops both outgoing and incoming routing updates. However, EIGRP still advertises the connected subnets if matched with an EIGRP network command.
# router eigrp 1
# passive-interface fastethernet0/0
Command to see list of passive-interfaces
# show ip protocols

20. What is the Feasibility Condition in EIGRP?
For any route to be a feasible successor it has to fulfill feasibility condition which is as follows:-
Advertised distance of Feasible successor should be less than Feasible distance of Successor
AD of feasible successor < FD of successor.

21. What is the Multicast IP address used by EIGRP?
EIGRP uses the multicast address 224.0.0.10